RS-485 along with other protocols allow for devices on a wired or wireless network to communicate with each other using a simple packet structure and protocol. Collision occurs when two devices transmit data at the same time on the same channel. In RS-485, a device decides to transmit only when there is quiet on the network transmission medium. While transmitting a packet, a device records what is received from the transmission medium, and if what is received is not the same as what was transmitted, the device determines that there was collision, and will retransmit the packet.
In many cases, RS-485 and similar protocols are used in applications in which a microcontroller or processor is responsible for transmission of a packet, and the time from detecting quiet on the network to the instant that packet transmission begins is great enough to significantly increase the opportunity for collision on the network. In other words, one device may detect quiet on the network and decide to transmit its data, and before it begins to do so, another device will detect quiet and decide to transmit its packet as well. This leads to collision, and in the case of protocols like RS-485 in which collision detection is performed by analyzing what was received on the network following packet transmission, the time period of a packet is lost before data can be successfully transmitted.